Method for preparing sintered product, sintered product and magnetostriction material

ABSTRACT

A method for manufacturing a sintered compact includes the steps of preparing an alloy powder having a composition represented by Expression 1: RT W  (where, R is at least one kind of rare earth metal, T is at least one kind of transition metal, and w defines a relation of 1&lt;w&lt;4), sintering the alloy powder in a vacuum atmosphere or an atmosphere containing gas with a molecular weight of 30 or less, and processing the alloy powder by a hot isostatic pressing. The sintered compact has a high density, and reduces deteriorations in its sintered compact properties such as magnetostrictive properties in an air atmosphere at high-temperatures.

TECHNICAL FIELD

The present invention relates to a manufacturing method that yield highdensity sintered compacts, sintered compacts with high density that canbe manufactured according to the manufacturing method, and tomagnetostrictive materials with superior magnetostrictive properties andideal for use on transducers between magnetic energy and mechanicalenergy.

BACKGROUND

When a ferromagnetic compact is magnetized, its dimensions change; thisphenomenon is called magnetostriction, and materials that cause thisphenomenon are called magnetostrictive materials. The saturationmagnetostrictive constant, which is the saturation change amount causedby magnetostriction, generally has the value of 10⁻⁵-10⁻⁶, andmagnetostrictive materials with large saturation magnetostrictiveconstants are widely used in oscillators, filters and sensors.

At present, magnetostrictive materials with even larger magnetostrictionvalues are demanded, and among the materials proposed are compounds madeof rare earth (R) and iron (Fe). R and Fe form RFe₂ Laves-typeintermetallic compounds, and although the magnetostriction value of RFe₂Laves-type intermetallic compounds is large when the external magneticfield is large, it is insufficient when the external magnetic field issmall. Consequently, a magnetostrictive material having an even largermagnetostriction value is in demand among RFe₂ Laves-type intermetalliccompounds.

As one technique to obtain a larger magnetostriction value for amagnetostrictive material, the density of sintered compact may beincreased when the sintered compact is manufactured using the powdermetallurgical method. The powder metallurgical method involves heatingmetal or alloy powder to high temperature and sintering the same, and itis a method to manufacture sintered compacts of magnetostrictivematerial having predetermined shapes. It is suitable for mass productionand offers an advantage of being able to produce a variety of shapeswith high yield.

However, the magnetostrictive material manufactured through the powdermetallurgical method has gaps among powder particles in themagnetostrictive material and the gaps remain and become pores aftersintering, and this phenomenon impede the manufacture of a high densitysintered compact. After using the material containing pores as amagnetostrictor for a long time, these pores lead to dry corrosion ofthe rare earth metal, which oxidizes, especially at high temperature inatmospheric air, and the magnetostrictive properties diminish with thesechanges.

One way proposed to reduce the number of pores in order to manufacturesintered compacts made of high density magnetostrictors is, for example,(1) to use an argon (Ar) gas atmosphere to sinter when manufacturing asupermagnetostrictor represented by RT₂ using the powder metallurgicalmethod (Croat, J. J. “Liquid Sintering of Rare Earth-Iron(Dy_(0.7)Tb_(0.3)Fe₂) Magnetostrictive Materials.” J. Appl. Phys. 49.3(1978)). Also, (2) there has been proposed a method for manufacturing amagnetostrictive material in which several types of raw material powdersobtained through machine grinding are sintered in an Ar gas atmosphere,in order to align the crystal orientation of the powders in a compactionprocess in magnetic field (see Japanese laid-open unexamined PatentApplication H 7-286249). Similarly, (3) another manufacturing methodproposed is a method for manufacturing a magnetostrictive material inwhich raw material powder that is composed of plural kinds of rawmaterials including hydrides of some of the raw materials is sintered inan Ar gas atmosphere.

However, the manufacturing method proposed by Croat yields only about86% density, which is low, in the sintered compact. Further, even withthe method for manufacturing a magnetostrictive material according tothe method described in Japanese laid-open unexamined Patent ApplicationH 7-286249, the density of the sintered compact formed from themagnetostrictive material is also low at only about 86%. Moreover, thedensity of the sintered compact obtained through the manufacturingmethod described above that uses several types of raw material powdersalso yields a low density of about 88-93%.

In terms of magnet materials, the following has been proposed: (4)permanent magnets with high coercive force that are manufactured byplastically forming a R—Fe—B system magnet material with a hot press(see Japanese laid-open unexamined Patent Application S 62-202506); (5)anisotropic magnetic powder in which a superplastic metal powder and apyrolytic binder are added to an anisotropic magnetic powder to form amixture, the mixture is oriented by a magnetic field, the binder iseliminated through pyrolysis, and the rest is subject to main sinteringand hot isostatic pressing (hereinafter called “HIP”) (see Japaneselaid-open unexamined Patent Application H 6-192709); and (6) R—Fe—Bsystem permanent magnets, in which permanent magnets are formed bycompacting raw material powders through compaction pressure and directlycirculating electric current; and raw material powder for permanentmagnets that can be compacted to have a high density, which aremanufactured by applying relatively a low pressure by means of hotpressing or HIP to the raw material powder (see Japanese laid-openunexamined Patent Application H 10-189319).

However, because the manufacturing methods described above involvesintering in Ar gas, which is an inert gas, the Ar gas fills the closedpores inside the sintered compacts, and when the sintered compacts areHIP-treated and compressed, the internal pressure caused by the Ar gasinside the closed pores leads to a strain; when these methods areapplied to magnetostrictive materials, the strain lowers magneticproperties such as magnetostriction value.

Another technique to increase the magnetostriction value ofmagnetostrictive material involves orienting an RFe₂ Laves-typeintermetallic compound manufactured under the powder metallurgicalmethod in the direction of a [111] axis, which is an easy axis ofmagnetization and which provides a large magnetostrictive constant, tothereby obtain a magnetostrictive material whose magnetostriction valueis large even when the external magnetic field is small and that hasgood magnetic field responsiveness.

For example, (7) among the conventional magnetostrictive materials inwhich crystals are oriented are magnetostrictive materials manufacturedthrough the single crystal method. In addition, there has been proposed(8) a magnetostrictive material oriented along a [111] axis through thepowder metallurgical method in which a Tb_(0.3)Dy_(0.7)Fe_(2.0) powderis compacted in a magnetic field and subsequently sintered (see U.S.Pat. No. 4,152,178). Furthermore, there has been proposed (9) an alloyof Dy, Tb and Fe in which particles of Fe₂Tb and Fe₂Dy are compacted ina magnetic field into a compression compact and sintered (see Japaneselaid-open unexamined Patent Application H 1-180943). Moreover, there isalso proposed (10) a magnetostrictive material in which a rareearth-iron with Mn added thereto is used as a basis and themagnetostrictive material is grown in a [110] axis orientation, which isan easy axis orientation for crystals to grow (see Japanese laid-openunexamined Patent Application H 5-148594).

Also, another proposed method relates to (11) a method to manufacture amagnetostrictive sintered compact, in which RFe₂ powder and powder of Rand Fe eutectic composition that is adjusted using the gas atomizingmethod or rotating electrode method are mixed, finely ground, compactedin a magnetic field and sintered (see Japanese laid-open unexaminedPatent Application H 6-256912). Further, a conventional technology thatinvolves finely grinding with a vibrating mill and sintering is known asa way to achieve high density in a sintered compact.

However, the single crystal method such as the method (7) describedabove, whether using the zone melting method or the Bridgman method,requires casting the raw material after melting it to form a cast ingot,making a single crystal using the cast ingot, annealing and machining;consequently, its productivity is low and because its shape is limitedto a cylindrical shape it requires cutting and other machining to makeit into articles. An additional problem with the single crystal method,particularly when using the Bridgman method, is that the single crystalfails to be oriented in the direction of the [111] axis. And theaforementioned method (8) described in U.S. Pat. No. 4,152,178 has aproblem of requiring a large magnetic field for orientation to takeplace due to the fact that the crystal magnetic anisotropy ofTb_(0.3)Dy_(0.7)Fe_(2.0) is small. Further, the alloy proposed in theaforementioned method (9) described in Japanese laid-open unexaminedPatent Application H 1-180943 entails a problem of the constituent metalcompounds failing to orient themselves in the direction of the [111]axis, since the easy axis of magnetization for Fe₂Tb is the [111] axis,while for Fe₂Dy it is the [100] axis. Also, in the aforementioned method(10) described in Japanese laid-open unexamined Patent Application H5-148594, due to the fact that the crystal grows in the direction of the[110] axis, the magnetostrictive material requires cutting and othermachining in order to obtain a magnetostrictive material oriented in the[111] axis, which is an easy axis of magnetization and provides thelargest magnetostrictive constant. Further, with the powder obtainedthrough the gas atomizing method in the aforementioned method (11)described in Japanese laid-open unexamined Patent Application H 6-256912or the powder obtained through the use of a vibrating mill describedabove, there is a problem in that the sintering density is notnecessarily sufficient for obtaining high magnetostrictive propertieseven though the sintering density is increased.

Furthermore, alloys comprising RFe₂ Laves-type intermetallic compoundsas described above sometimes precipitate, depending on the alloycomposition and/or manufacturing conditions, heterogeneous phases suchas phases represented by RFe₃, for example, and/or phases formed byimpurities in raw material, such as oxides or carbides, in addition tothe main phase RFe₂. These heterogeneous phases affect themagnetostrictive properties of RFe₂ Laves-type intermetallic compounds.Consequently, the precipitation of heterogeneous phases must becontrolled in order to obtain superior magnetostrictive properties andto prevent fluctuations in the properties among products, i.e.,magnetostrictive materials.

A “super magnetostrictive alloy” described in Japanese laid-openunexamined Patent Application H 5-148594 is an alloy of Fe and R thathas been partially replaced with Mn and other metals, and is an alloycontaining 5 vol. % or less of the RFe₃ phase, which is a heterogeneousphase. By controlling the alloy composition in the supermagnetostrictive alloy, the precipitation of the RFe₃ phase isrestricted and the magnetostrictive properties of the alloy areimproved.

However, the aforementioned Japanese laid-open unexamined PatentApplication H 5-148594 provides no consideration as to systems in whichthe amount of rare earth metal represented by R is increased in thealloy composition. Accordingly, it is necessary to study suchcompositions and find the optimum range.

DISCLOSURE OF INVENTION

The present invention was conceived in view of the above problems, andits object is to provide a method for manufacturing a fine sinteredcompact with high density. In addition, another object is to provide amethod for manufacturing a sintered compact with little internal strain,which results from closed pores with low internal pressure.

Furthermore, in accordance with a further object, by using thesemanufacturing methods for fine sintered compacts with high density, thepresent invention provides magnetostrictive materials with largemagnetostriction values, small reduction in magnetostriction value overtime, and without cracks. Moreover, it is another object to provide asuperior magnetostrictive material in which the crystal orientation isaligned to gain a large magnetostriction value, the precipitation ofheterogeneous phases is controlled to prevent change in properties amongproducts, and that has high productivity.

To solve the problems described above, the present invention recited inclaim 1 pertains to a method for manufacturing a sintered compactincludes the step of sintering in a mixed atmosphere of hydrogen gas andinert gas an alloy powder having a composition represented by Expression1: RT_(W) (where, R is at least one kind of rare earth metal, T is atleast one kind of transition metal, and w defines a range of 1<w<4).

In accordance with the present invention recited in claim 2, in themethod for manufacturing a sintered compact recited in claim 1, themixed atmosphere may be an atmosphere in a temperature heating processthat is conducted at 650° C. or higher and/or in a stable temperaturestate that is conducted in a range from 1150° C. to 1230° C., containinghydrogen gas and argon (Ar) gas represented by Expression 2: Hydrogengas:Argon (Ar) gas=X:100−X, wherein X (vol. %) is 0<X<50.

The present invention recited in claim 3 pertains to an alloy powderhaving a composition represented by Expression 1: RT_(W) (where, R is atleast one kind of rare earth metal, T is at least one kind of transitionmetal, and w defines a range of 1<w<4), which is sintered in a vacuumatmosphere or an atmosphere containing gas with a molecular weight of 30or less, and processed by a hot isostatic pressing.

In accordance with the present invention recited in claim 4, in themethod for manufacturing a sintered compact recited in claim 3, anaverage grain size of the alloy powder may be in a range from 10 μm to30 μm.

In accordance with the present invention recited in claim 5, in themethod for manufacturing a sintered compact recited in claim 3 or claim4, the atmosphere at the time of sintering may contain at least one gasselected from He gas, Ne gas and hydrogen gas.

In accordance with the present invention recited in claim 6, in themethod for manufacturing a sintered compact recited in any one of claim3 through claim 5, the atmosphere during the hot isostatic pressingprocess may be a vacuum.

In accordance with the present invention recited in claim 7, in themethod for manufacturing a sintered compact recited in any one of claim3 through claim 6, the atmosphere during the sintering in thetemperature heating process that is conducted at 600° C. or higher or inthe stable temperature state may be an atmosphere containing hydrogengas and argon (Ar) gas represented by Expression 3: Hydrogen gas:Argon(Ar) gas=Y:100−Y, wherein Y (vol. %) is Y>50.

In accordance with the present invention recited in claim 8, the methodfor manufacturing a sintered compact recited in any one of claim 3through claim 7 may further include the step of annealing the alloypowder in an Ar gas atmosphere that is conducted in a range from 1150°C. to 1230° C., after the sintering and the hot isostatic pressingprocess.

In accordance with the present invention recited in claim 9, in themethod for manufacturing a sintered compact recited in any one of claim3 through claim 8, the sintered compact may have a relative density of98% or greater.

In accordance with the present invention recited in claim 10, in themethod for manufacturing a sintered compact recited in any one of claim1 through claim 9, R may be at least one rare earth metal selected fromthe group consisting of Nd, Pr, Sm, Tb, Dy and Ho.

In accordance with the present invention recited in claim 11, in themethod for manufacturing a sintered compact recited in claim 10, R maybe composed of Tb and Dy.

In accordance with the present invention recited in claim 12, in themethod for manufacturing a sintered compact recited in claim 11, R mayhave a composition that is represented by Expression 4: Tb_(v)Dy_(1-v),where v is in a range of 0.27≦v≦0.50.

In accordance with the present invention recited in claim 13, in themethod for manufacturing a sintered compact recited in any one of claim1 through claim 12, T may be at least one element selected from Fe, Coand Ni.

The present invention recited in claim 14 pertains to a magnetostrictivematerial that is represented by Expression 5: RT_(W) (where, R is atleast one kind of rare earth metal, T is at least one kind of transitionmetal, and w defines a relation of 1.50≦w≦2.30) may be formed bycompaction in a magnetic field, wherein the magnetostrictive materialhas a degree of orientation of a [111] axis in a direction in parallelwith the magnetic field given by Formula (1):

$\begin{matrix}{{{{Degree}\mspace{14mu}{of}\mspace{14mu}{orientation}} = \frac{{I(222)}{(//)/{I(311)}}(//)}{{I(222)}{(\bot)/{I(311)}}(\bot)}},} & {{Formula}\mspace{14mu}(1)}\end{matrix}$which is 2.0 or greater (where, each of the I (222) and I (311)represents an x-ray diffraction intensity on a (222) plane and a (311)plane, respectively, and (//) and (⊥) represent measurements taken on aplane parallel and on a plane vertical, respectively, with respect to amagnetic field orientation in the compaction in magnetic field).

In accordance with the present invention recited in claim 15, in themagnetostrictive material recited in claim 14, the degree of orientationof the magnetostrictive material in which the [111] axis orients in adirection in parallel with the magnetic field may have a value of 7.0 orgreater given by Formula (1).

The present invention recited in claim 16 pertains to a magnetostrictivematerial that is represented by Expression 5: RT_(W) (where, R is atleast one kind of rare earth metal, T is at least one kind of transitionmetal, and w defines a range of 1.50≦w≦2.30), which is formed bycompaction in a magnetic field, the magnetostrictive material having astructure composed of a RT₂ main phase and at least one kind ofheterogeneous phase including a phase having R as a main composition.

In accordance with the present invention recited in claim 17, in themagnetostrictive material recited in claim 16, the ratio of the phasehaving R as a main composition among the heterogeneous phase to the RT₂main phase ([R]/[RT₂]) may be in a range of 0<[R]/[RT₂]≦0.45.

In accordance with the present invention recited in claim 18, in themagnetostrictive material recited in any of claim 14 through claim 17,the magnetostrictive material may be represented by Expression 6:(Tb_(v)Dy_(1-v)) T_(w) (where, v and w are atom ratios wherein v and ware in ranges of 0.27≦v≦0.50 and 1.50≦w≦2.30, respectively.)

In accordance with the present invention recited in claim 19, in themagnetostrictive material recited in any of claim 14 through claim 18, Tin the magnetostrictive material may be at least one kind of metalselected from Fe, Ni and Co.

In accordance with the present invention recited in claim 20, in themagnetostrictive material recited in any of claim 14 through claim 19,the magnetostrictive material may be formed by compacting in a magneticfield and then sintering a mixture containing a material A that isrepresented by Expression 7: (Tb_(x)Dy_(1-x)) T_(y) (where, x and y areatom ratios wherein x and y are in ranges of 0.35<x≦0.50 and1.50≦y≦2.30, respectively), a material B that is represented byExpression 8: Dy_(t) T_(1-t) (where, Dy may include at least one of Tband Ho, and t is an atom ratio in a rage of 0.37≦t≦1.00), and a materialC containing T.

In accordance with the present invention recited in claim 21, in themagnetostrictive material recited in claim 20, the material Brepresented by Expression 8 in the magnetostrictive material containshydrogen in the amount of 7,000 ppm or greater but 22,000 ppm or lower.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an x-ray diffraction graph of a plane parallel to a directionin which a magnetostrictive material in accordance with the presentinvention is formed by compaction in magnetic field.

FIG. 2 is an x-ray diffraction graph of a plane perpendicular to adirection in which the magnetostrictive material in accordance with thepresent invention is formed by compaction in a magnetic field.

FIG. 3 is a graph indicating the relationship between the ratio of x-raydiffraction intensity on a (222) plane to that on a (311) plane, and themagnetostriction value, in the parallel direction and the verticaldirection.

FIG. 4 is a graph indicating the relationship between the degree oforientation and the magnetostriction value λ_(1.0) in the verticaldirection.

FIG. 5 is a drawing showing the relationship between [R]/[RT₂] and themagnetostriction value of a (Tb, Dy) T_(w) alloy.

FIG. 6 shows a manufacturing process for manufacturing amagnetostrictive material according to the present invention.

FIGS. 7( a) and 7(b) show the temperature (° C.) of heat treatmentagainst time passed (in minutes) and the density of the sintered compact(%) against time passed (in minutes), respectively.

FIG. 8 is a graph showing the density and magnetostriction value of asintered compact in which a hydrogen storage material was used and whichwas sintered as the content of hydrogen gas was varied between 0 and100%.

FIG. 9 is a graph showing the density and magnetostriction value of asintered compact in which a non-hydrogen storage material was used andwhich was sintered as the content of hydrogen gas was varied between 0and 100%.

FIG. 10 is a graph showing the density and magnetostriction value of asintered compact sintered as the atmosphere was varied from an Ar gasatmosphere to a mixed atmosphere of 35 vol. % hydrogen gas and 65 vol. %Ar gas.

FIG. 11 is a graph showing the density and magnetostriction value of asintered compact sintered by varying the hydrogenation endingtemperature, at which point the shift from a mixed atmosphere of 35 vol.% hydrogen gas and 65 vol. % Ar gas to an atmosphere of Ar gas alonetakes place.

FIG. 12 is a graph showing the deterioration rate of magnetostrictionvalue when each of the sintered compacts is let stand for 1000 hours inatmospheric air.

FIG. 13 is a graph showing heat treatment conditions for sinteringaccording to the present invention.

FIG. 14 is a graph showing HIP treatment conditions according to thepresent invention.

FIG. 15 is a graph showing heat treatment conditions for annealingaccording to the present invention.

FIGS. 16( a), 16(b) and 16(c) show photographs of closed pores formedwhen sintering, HIP and annealing treatments are conducted in a vacuumatmosphere, respectively.

FIGS. 17( a), 17(b) and 17(c) show photographs of closed pores formedwhen sintering, HIP and annealing treatments are conducted in an Ar gasatmosphere, respectively.

FIGS. 18( a), 18(b) and 18(c) show photographs of closed pores whensintering, HIP and annealing treatments are conducted in a hydrogen gasatmosphere, respectively.

FIGS. 19( a), 19(b) and 19(c) show photographs of closed pores formedwhen sintering, HIP and annealing treatments are conducted in a mixedgas atmosphere of hydrogen:Ar=65:35, respectively.

FIGS. 20( a), 20(b) and 20(c) show photographs of closed pores formedwhen sintering, HIP and annealing treatments are conducted in a mixedgas atmosphere of hydrogen:Ar=50:50, respectively.

FIGS. 21( a), 21(b) and 21(c) show photographs of closed pores whensintering, HIP and annealing treatments are conducted in a mixed gasatmosphere of hydrogen:Ar=35:65, respectively.

FIG. 22 is an SEM photograph in cross section of a magnetostrictivematerial obtained from Embodiment Example 9.

FIG. 23 is an SEM photograph in cross section of a magnetostrictivematerial obtained from Embodiment Example 10.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described in detail below.

A first method for manufacturing sintered compacts in accordance with anembodiment of the present invention involves sintering alloy powdershaving a composition represented by Expression 1: RT_(w) (where R is oneor more types of rare earth metals, T is one or more types of transitionmetals, and w is 1<w<4).

Here, R represents one or more types of rare earth metals selected fromamong lanthanoid series and actinoid series, and Y. Of these, rare earthmetals Nd, Pr, Sm, Tb, Dy and Ho are preferable as R, and Tb and Dy areparticularly preferable. This is due to the fact that each of the RT₂Laves-type intermetallic compounds formed by Tb and/or Dy has a highCurie temperature and a large magnetostriction value. In addition, R maybe a mixture of these rare earth metals.

T represents one or more types of transition metals. Of these,transition metals Fe, Co, Ni, Mn, Cr and Mo are preferable as T, and Fe,Co and Ni are particularly preferable; these metals may be combined foruse.

In alloys represented by Expression 1: RT_(w), w is 1<w<4. Each of theRT₂ Laves-type intermetallic compounds formed by R and T in which w=2 inExpression 1 has a high Curie temperature and a large magnetostrictionvalue, and therefore is suitable as a magnetostrictor. When w is 1 orless, an RT phase would precipitate in the heat treatment aftersintering and the magnetostriction value would fall. When w is 4 orhigher, there would be more RT₃ phase or RT₅ phase, which would reducethe magnetostriction value. Consequently, w in the range of 1<w<4 isideal to obtain phases rich in RT₂.

R may be a mixture of rare earth metals, and a mixture of Tb and Dy isparticularly favorable. Further, in alloys represented by Expression 4:Tb_(v)Dy_(1-v), it is preferable that v is in the range of 0.27≦v≦0.5.With this, each of the alloys (Tb_(v)Dy_(1-v)) T_(w) having a largesaturation magnetostrictive constant and a large magnetostriction valuecan be obtained. When v is less than 0.27, sufficient magnetostrictionvalues cannot be obtained in a temperature range lower than roomtemperature, and when v is more than 0.50, sufficient magnetostrictionvalue cannot be obtained in room temperature range.

Fe is particularly preferable as T, and Fe can form a (Tb, Dy) Fe₂intermetallic compound with Tb and Dy, which would yield a sinteredcompact having a high magnetostriction value and high magnetostrictiveproperties. Part of Fe may be replaced with Co or Ni. When these replacepart of Fe, Co increases magnetic anisotropy but reduces magneticpermeability, while Ni lowers the Curie temperature, and bothconsequently reduce the magnetostriction value in a condition of roomtemperature and high magnetic field; thus, Fe should constitute 70 wt. %or more, and preferably 80 wt. % or more.

In addition, it is preferable in the first method for manufacturingsintered compact to contain as part of the alloy powder a raw materialthat can be treated to absorb hydrogen. Having the alloy powder absorbhydrogen causes strain, and the internal stress of the strain leads tocracks. Consequently, pressure is applied on the mixed alloy powder whenforming a green compact, and the metal alloy powder becomes groundinternally in a mixed state and becomes a fine powder, so that whensintered it can yield a fine and high density sintered compact.Furthermore, rare earths Tb and Dy are easily oxidized and form on theirsurfaces oxide films with a high melting point, which restrict progressin sintering, in the presence of even a small amount of oxygen, but Tband Dy become more difficult to be oxidized when they absorb hydrogen.As a result, a high density sintered compact can be manufactured byhydrogen absorbing part of the alloy powder.

The first method for manufacturing sintered compact involves sinteringthe raw material powder of the alloy represented by RT_(w) in atemperature zone of 650° C. or higher during a temperature heatingprocess and/or a stable temperature zone of between 1150° C. and 1230°C., and in a mixed atmosphere of hydrogen gas and inert gas.

Sintering is conducted by raising the temperature of the compacted rawmaterial powder inside an oven for heat treatment. The rate for heatingtemperature is 3-20° C./minute. Rates lower than 3° C./minute lead tolow productivity, and rates higher than 20° C./minute cause thetemperature of the compacted raw material powder inside the oven to beuneven, which causes segregations and heterogeneous phases to appear.The temperature heating process should be at 650° C. or higher toprevent oxidation caused by a minuscule amount of residual oxygen.

Sintering is also conducted at a stable temperature in which thetemperature is maintained at a virtually constant temperature level. Thepreferable stable temperature is in the range of 1150° C. to 1230° C.When the stable temperature is less than 1150° C., a long time isrequired to remove internal strains and therefore not efficient, whilewhen the stable temperature is higher than 1230° C., the sinteredcompact can fuse since the temperature approaches the melting point ofthe alloy represented by RT_(w) and heterogeneous phases such as RT₃phase can precipitate.

The atmosphere preferable for sintering is a mixed atmosphere ofhydrogen gas and an inert gas. R reacts extremely easily with oxygen andforms a stable rare earth oxide. Such an oxide has low magnetism butdoes not indicate magnetic properties that would make it a practicalmagnetic material. Since even a small amount of oxygen can drasticallyreduce the magnetic properties of a sintered compact in high temperaturesintering, an atmosphere containing hydrogen gas is especiallypreferable for heat treatments such as sintering. An atmosphere thatprevents oxidation is an inert gas atmosphere, but an inert gas alonecannot completely remove oxygen and rare earth metals with highreactivity with oxygen form oxides in such an atmosphere; consequently,a mixed atmosphere of hydrogen gas and inert gas is preferable toprevent such oxidation.

A mixed atmosphere of hydrogen gas and inert gas is preferably asrepresented by Expression 2: hydrogen gas:argon (Ar) gas=X:100−X, whereX (vol. %) is 0<X<50. Ar gas is an inert gas that does not oxidize R,and it therefore mixes with hydrogen gas to create an atmosphere havinga reduction action. In order to obtain the reduction action, X (vol. %)should be at least 0<X. Further, since the reduction action becomessaturated when X (vol. %) is 50≦X, X<50 is preferable.

It is further preferable for the mixed atmosphere of hydrogen gas and Argas to be in a temperature zone of 650° C. during temperature heatingprocess or to be in a stable temperature zone.

A second method for manufacturing sintered compacts in accordance withan embodiment of the present invention involves sintering alloy powdershaving a composition represented by Expression 1: RT_(w) (where R is oneor more types of rare earth metals, T is one or more types of transitionmetals, and w is 1<w<4) in vacuum atmosphere or in an atmospherecontaining a gas whose molecular weight is 30 or less, and conducting anHIP treatment.

The description concerning alloys represented by Expression 1: RT_(w) isthe same as the one for the first method for manufacturing sinteredcompacts and is therefore omitted here.

The average particle size of the alloy powders represented by RT_(w)should be between 10 μm and 30 μm, and preferably between 20 μm and 30μm. In normal sintering that takes place in an atmosphere of inert gassuch as Ar or nitrogen, each of the alloy powders whose average particlesize is 10 μm or greater has small surface energy due to its largeparticle size and normally has difficulties making progress in sinteringeven when heat treated and also has difficulties in attaining highdensity, but with HIP treatment it can make progress in sintering evenwhen its average particle size is large. When the average particle sizeis less than 10 μm the surface area becomes large, the powder oxidizeswhen ground and forms an oxide film on the surface of the powder; thisimpedes the powder from making progress in sintering so that the densityof the sintered compact fails to increase. When the average particlesize exceeds 30 μm, the small surface energy makes it difficult for thepowder to make progress in sintering, so that the density of thesintered compact fails to increase and the failure of the sintering toprogress causes an increased number of open and closed pores to developinside.

The heat treatment conditions for sintering are temperature of between1150° C. and 1230° C. and heat treatment time of one to ten hours. Attemperatures lower than 1150° C., there is insufficient crystal growthof the sintered compact, and at over 1230° C., the sintered compactturns into a fused state.

The atmosphere for sintering is preferably a vacuum atmosphere or anatmosphere containing a gas, particularly hydrogen gas or an inert gassuch as Ne or He, whose molecular weight is 30 or less. By sintering ina vacuum atmosphere, the oxidation of alloy powder can be prevented.Even when there are closed pores, no internal pressure develops sincethere is no residual gas in the closed pores, and this prevents themagnetostriction value from falling.

Additionally, in an atmosphere containing a gas with low molecularweight, particularly an inert gas such as Ne or He whose molecularweight is 30 or less, oxidation of the alloy powder can be prevented.Low molecular weight gases such as Ne and He can be dissipated outsidethe sintered compact through the grain boundary of the sintered compacteven when the gases fill the closed pores.

And in an atmosphere containing hydrogen, the sintered compact does notform any oxide films due to the reduction action of the hydrogen. Inaddition, hydrogen is absorbed by the alloy represented by Expression 1:RT_(w), so that it becomes easy for hydrogen to penetrate inside or toform hydrides. As a result of this, even when there is residual hydrogenin closed pores the hydrogen travels as it forms hydrides and ultimatelyreaches the exterior surface of the sintered compact and becomesdissipated outside. Consequently, hydrogen gas in the atmosphere doesnot remain as a residue in closed pores that form inside the sinteredcompact.

The atmosphere favorable for heat treatment when the temperature reaches600° C. or higher during the temperature heating process or when thestable temperature is 1100° C. or higher is a mixed gas atmosphere ofhydrogen and Ar represented by Expression 3: hydrogen:Ar=Y:100−Y, whereY is Y>50 (vol. %). Because Ar's molecular weight is 39, which is over30, it is difficult for Ar to pass through the grain boundary and thebody of the sintered compact. However, when hydrogen constitutes 50 vol.% or more, even when the atmosphere's mixed gas remains as a residue inclosed pores hydrogen dissipates and vanishes so that it does not remainas a residue in the closed pores inside; consequently, there is nostrain caused by internal stress and the magnetostriction value of themagnetostrictive material obtained is not diminished. As a result, gasessuch as He, Ne or hydrogen that dissipate outside can be used to form amixed gas with Ar. A mixed gas containing hydrogen is especiallyfavorable. This is due to the fact that hydrogen has a fast dissipationspeed.

The reason for using a mixed gas atmosphere having a hydrogen and Arcomposition as described above for heat treatment when the temperaturereaches 600° C. or higher during the temperature heating process or whenthe stable temperature is 1100° C. or higher is to create an agreeablecondition for sintering to proceed as the R and T metals of the alloyrepresented by RT_(w) soften and the powder particles begin toelastically deform, as well as to prevent oxidation by a minusculeamount of residual oxygen. The amount of hydrogen contained in thesintered compact should be 2500 wt. ppm at most, and preferably 2000 wt.ppm or less. When an oxide film forms on the surface of the alloy powderdue to oxidation, sintering is hindered and the density of the sinteredcompact fails to increase. In addition, oxygen becomes dispersed insidethe magnetostrictive material to make the material an impurity-dispersedmaterial, which decreases its magnetic properties such asmagnetostriction value and magnetic permeability.

Further in the second method for manufacturing a sintered compact, anHIP treatment is conducted after sintering. In order to apply anisostatic pressure, an HIP apparatus is used. In the HIP treatment, apressure is applied equally from all directions. By applying anisostatic pressure, the density of the sintered compact can be increasedand the number of closed and open pores inside the sintered compact canbe reduced. In addition, the density inside the sintered compact can bemade uniform.

Furthermore, the HIP treatment may be conducted in vacuum. By performingthis treatment in vacuum, oxidation of the alloy powder is prevented.Moreover, compared to applying a pressure from one direction, applyingan isostatic pressure in vacuum makes the movement of Ne, hydrogen andother similar atoms easier, and this results in a higher density andfiner sintered compact.

The second method for manufacturing a sintered compact achieves arelative density of 98% or more for the sintered compact as a result ofthe HIP treatment described above. When the relative density is lessthan 98%, the proportion of closed and open pores is higher, whichcauses dry corrosion of R to develop and therefore oxidation, which inturn causes magnetic properties such as magnetostriction value to fallafter a long term use. Furthermore, when the relative density is low andthere are numerous pores, stress caused by a long term use leads tocracks. The relative density of the sintered compact should be 98% ormore in order to prevent these from occurring.

It is preferable to anneal in an Ar gas atmosphere at a temperaturebetween 1150° C. and 1230° C. after sintering and HIP treatment. This isto remove internal strains in the sintered compact that were caused bythe HIP treatment. At temperatures under 1150° C., a long time isrequired to remove internal strains and therefore the anneal treatmentis not efficient. At temperatures over 1230° C., the sintered compactcan fuse since the temperature approaches the melting point of the alloyrepresented by Expression 1: RT_(w) and other phases such as RT₃ phasecan precipitate. And when a gas in the atmosphere used during sinteringor a gas generated from decomposition of components of the sinteredcompact fills the closed pores, the compression that takes place duringthe HIP treatment reduces the volume of the closed pores, while at thesame time the gas pressure increases. When the sintered compact is thenheated in annealing, gas expands and can cause cracks to develop on thesintered compact. However, by using gases whose molecular weights are ineach case 30 or less according to the present invention, no gases fromsintering remain as residue, so that no cracks result from annealing andinternal strains can be removed. Further, by removing internal strains,magnetic properties such as magnetostriction value can be improved.

Next, the magnetostrictive materials according to the present inventionare represented by Expression 5: RT_(w). Here, R includes one or moretypes of rare earth metals selected from among lanthanoid series andactinoid series, and Y. Of these, rare earth metals Nd, Pr, Sm, Tb, Dyand Ho are preferable as R, and Tb and Dy are particularly preferable.These rare earth metals may be used mixed.

T represents at least one type of metal selected from among Fe, Co andNi. In addition, transition metals that form alloys with a rare earthmetal R may be included in T. A list of transition metals specificallyincludes Mn, Cr, Mo and W.

In Expression 5, w represents atomic ratio and is 1.50≦w≦2.30. Each ofthe RT₂ Laves-type intermetallic compounds formed when w=2 in RT_(w) hasa high Curie temperature and a large magnetostriction value, andtherefore is suitable as a magnetostrictive material. In order to forman alloy whose main phase is RT₂, w should be in the range describedabove. In compositions in which w is less than 1.50, phases whoseprimary component is R become abundant, which causes a drastic decreasein magnetostriction value, while in compositions in which w is more than2.30, phases rich in T such as RT₃ become abundant, which causes adecrease in magnetostriction value; consequently neither is favorable.

The magnetostrictive materials according to the present invention andrepresented by Expression 5: RT_(w) more preferably contain Tb and Dy asR and are magnetostrictive materials represented by Expression 6:(Tb_(v)Dy_(1-v)) T_(w). Here, T is at least one type of metal selectedfrom among Fe, Co and Ni, as described above. Fe is especiallypreferable since it forms with Tb and Dy a (Tb, Dy) Fe₂ intermetalliccompound that has high magnetostrictive properties. Part of Fe may bereplaced with Co or Ni, but Co increases magnetic anisotropy whilereducing magnetic permeability, while Ni lowers the Curie temperature,and both consequently reduce the magnetostriction value in a conditionof room temperature and high magnetic field; thus, Fe should constitute70 wt. % or more, and preferably 80 wt. % or more. In addition, T mayinclude a transition metal that forms an alloy with rare earth metalsTb, Dy or Ho. A list of transition metals specifically includes Mn, Cr,Mo and W.

In Expression 5, w is 1.50≦w≦2.30, as described earlier, and v is0.27≦v≦0.5. When v is less than 0.27, sufficient magnetostriction valuecannot be obtained in a temperature range lower than room temperature,and when v is more than 0.50, sufficient magnetostriction value cannotbe obtained in room temperature range.

Each of the magnetostrictive materials according to the presentinvention has the composition described above and its degree oforientation in a [111] axis is 2.0 or more as a value of formula (1):

$\begin{matrix}{{{{Degree}\mspace{14mu}{of}\mspace{14mu}{orientation}} = \frac{{I(222)}{(//)/{I(311)}}(//)}{{I(222)}{(\bot)/{I(311)}}(\bot)}},} & {{Formula}\mspace{14mu}(1)}\end{matrix}$

Here, each of the I (222) and I (311) represents x-ray diffractionintensity on a (222) plane and a (311) plane, respectively, and (//) and(⊥) represent measurements taken on a plane parallel and on a planevertical, respectively, to the direction of a magnetic field whenconducting compaction in magnetic field.

Each of the magnetostrictive materials according to the presentinvention is a Laves-type intermetallic compound whose easy axis ofmagnetization is a [111] axis direction and whose magnetostrictiveconstant is largest in the [111] axis direction. Accordingly, themagnetostrictive material is oriented in a direction parallel to themagnetic field in the compaction process conducted in a magnetic field,and has the degree of orientation being 2.0 or more.

The degree of orientation is obtained by measuring the x-ray intensitydiffracted on a (222) plane, where the x-ray diffraction is strongest,and on a (311) plane, where the impact from the measurement noise issmall, and expressed as a ratio of the two.

FIG. 1 is a graph of x-ray diffraction measured on a plane parallel to adirection in which a magnetostrictive material according to the presentinvention is formed by compaction in a magnetic field. The large peakindicates the intensity of the x-ray that is diffracted on the (222)plane. The small peak indicates the intensity of the x-ray thatdiffracted on the (311) plane.

FIG. 2 is a graph of x-ray diffraction on a plane vertical to thedirection in which the magnetostrictive material according to thepresent invention is formed by compaction in a magnetic field. The largepeak indicates the intensity of the x-ray that diffracted on the (311)plane. The small peak indicates the intensity of the x-ray thatdiffracted on the (222) plane.

FIG. 3 is a graph showing the relationship between the ratio of x-raydiffraction intensity on the (222) plane to x-ray diffraction intensityon the (311) plane, and the magnetostriction value, in parallel andvertical directions. The magnetostriction value λ is measured byapplying a magnetic field to a sample and using a strain gauge tomeasure the strain that varies parallel to the magnetic field applied.Here, the magnetostriction value λ_(1.0) (ppm) is 8×10⁴ A/m (1.0 kOe),which is the value of the magnetic field applied.

As shown in FIG. 3, the magnetostriction value λ_(1.0) (ppm) growslarger as the ratio (I (222) (//)/I (311) (//)) of x-ray diffractionintensities in the direction parallel to the magnetic field applied inthe compaction in a magnetic field grows larger. On the other hand, themagnetostriction value λ_(1.0) (ppm) hardly changes against the value ofthe ratio (I (222) (⊥)/I (311) (⊥)) of x-ray diffraction intensities inthe direction vertical to the magnetic field applied in compaction in amagnetic field. This is due to the fact that the proportion oriented inthe vertical direction is small, which causes the changes to be small aswell.

FIG. 4 is a graph showing the relationship between the degree oforientation and the magnetostriction value λ_(1.0) in the verticaldirection. As shown in FIG. 4, the magnetostriction value λ_(1.0) growslarger as the degree of orientation grows larger. For themagnetostrictive material according to the present invention, themagnetostriction value λ_(1.0) should preferably be 700 ppm or more.This is due to the fact that the magnetostriction value λ_(1.0) of anisotropic magnetostrictive material that does not allow orientation andthat has the same composition as the magnetostrictive material accordingto the present invention is 600-700 ppm, and the magnetostriction valueλ_(1.0) of the magnetostrictive material according to the presentinvention must be larger than this value. Accordingly, as shown in FIG.4, the degree of orientation should be 2.0 or more so that themagnetostriction value that is practical as a displacement amount is 700ppm or more in a magnetic field of 8×10⁴ A/m (1.0 kOe).

Further, a zircon titanic acid (PZT) piezoelectric element, which has alarge drive voltage and slow response speed, can gain a displacementamount of 800-900 ppm, depending on the electric field, in contrast toconventional ferrite magnetostrictive materials. However, the ferritemagnetostrictive materials' displacement amount due to a magnetic fieldis only about 30 ppm for practical purposes. Consequently, a materialwith a small drive voltage, fast response speed and whose displacementamount caused by electric and magnetic fields is 900 ppm or more issought, and to this end it is even more preferable for the degree oforientation to be 7.0 or more in the magnetostrictive material accordingto the present invention so that its displacement amount would be atleast 900 ppm.

On the other hand, each of the magnetostrictive materials represented byExpression 5: RT_(w), or preferably by Expression 6: (Tb_(v)Dy_(1-v))T_(w), has a structure consisting of a main phase RT₂ and one or moretypes of heterogeneous phases including a phase whose primary componentis R. Heterogeneous phases are phases whose compositions are other thanthe composition of the main phase RT₂, and include phases rich in T suchas RT₃, oxide phases and impurity phases, in addition to phases whoseprimary components are R. As described earlier, the proportion ofheterogeneous phases such as phases whose primary components are R andRT₃ varies depending on the value of w in RT_(w). Based on the above,although it is true that it is not favorable for the structure tocontain large amounts of such heterogeneous phases, phases whose primarycomponents are R act to prevent the main phase RT₂ from becomingoxidized, because R has a property that is readily oxidized, andtherefore is prone to react with oxygen that exist in minuscule amountsduring manufacturing processes.

It is favorable that phases whose primary components are R exist in themagnetostrictive materials in the following quantitative ratio:0<[R]/[RT₂]≦0.45, where [R]/[RT₂] is the ratio of phases whose primarycomponents are R to the main phase RT₂.

FIG. 5 shows the relationship between [R]/[RT₂] and the magnetostrictionvalue of the (Tb, Dy) T_(w) alloy. The horizontal axis ([R]/[RT₂]) showsratio values of areas of the respective phases observed on scanningelectronic microscope (SEM) photographs. The vertical axis shows themagnetostriction value (ppm) in a 8×10⁴ A/m (1.0 kOe) magnetic field. Asshown in FIG. 5, when the value of [R]/[RT₂] is zero, themagnetostriction value is not a large value, but once the value of[R]/[RT₂] exceeds zero and increases slightly, there is a rapid rise inthe magnetostriction value. Subsequently, the magnetostriction valuefalls as the value of [R]/[RT₂] increases further.

This phenomenon can be explained as follows: the area immediatelyfollowing the point where the value of [R]/[RT₂] exceeds zero is an areawhere there is the largest amount of the RT₂ phase in the alloy, and themagnetostriction value is therefore extremely high in this area. As thevalue of [R]/[RT₂] increases, i.e., as the volume of phases whoseprimary components are R in the alloy increases, the magnetostrictiveproperties of the RT₂ phase gradually diminish. As a result, it ispreferable that the value of [R]/[RT₂] is 0.45 or less to correspond toan area in which the alloy shows magnetostrictive properties higher thanthose of the isotropic magnetostrictive material.

On the other hand, the reason for the magnetostriction value to besignificantly low when the value of [R]/[RT₂] is near zero is thatalthough there are less precipitations of phases whose primarycomponents are R in the alloy, there are precipitations of phases richin T such as RT₃ phases.

Consequently, by setting the value of [R]/[RT₂] in the range of0<[R]/[RT₂]≦0.45, an magnetostriction value larger than that of theisotropic magnetostrictive material can be obtained in a stable manner.More preferably, the range should be 0.001≦[R]/[RT₂]≦0.30. This is therange that, in the example of the (Tb, Dy) T_(w) alloy in a 8×10⁴ A/m(1.0 kOe) magnetic field in FIG. 5, corresponds to an area that shows astable and high magnetostriction value of 800 ppm or more. And even morepreferably, the range should be 0.007≦[R]/[RT₂]≦0.094. In FIG. 5, thisis the range that corresponds to an area that shows an extremely highmagnetostriction value of 1100 ppm or more.

Next, we will describe the method for manufacturing the magnetostrictivematerial according to the present invention. FIG. 6 shows themanufacturing processes for the magnetostrictive material according tothe present invention.

As FIG. 6 shows, the magnetostrictive material according to the presentinvention involves grinding and mixing powder raw materials A, B and C,and compacting them in a magnetic field.

For the raw material A, a raw material represented by Expression 7:(Tb_(x)Dy_(1-x)) is used. Here, T is at least one type of metal selectedfrom among Fe, Co and Ni, and may be Fe alone. This is because Fe formswith Tb and Dy a (Tb, Dy) Fe₂ intermetallic compound that has highmagnetostrictive properties. Part of Fe may be replaced with Co or Ni,but Co increases magnetic anisotropy but reduces magnetic permeability,while Ni lowers the Curie temperature, and both consequently reduce themagnetostriction value in a condition of room temperature and highmagnetic field; thus, Fe should constitute 70 wt. % or more, andpreferably 80 wt. % or more. In addition, transition metals that formalloys with rare earth metals Tb, Dy or Ho may be included in theselection. A list of transition metals specifically includes Mn, Cr, Moand W.

Part of Tb in the raw material A may be replaced with rare earths (R′)that exclude Dy and Ho. A list of R′, for example, includes Nd, Pr, Gdand Y.

x and y in Expression 7 represent atomic ratios, where 0.35<x≦0.50 and1.50≦y≦2.30. The reason for such a composition is as follows: in orderto further enhance magnetostrictive properties of a magnetostrictivematerial, it is effective to give anisotropy to the material byorienting the crystal axis having the direction that presents a largemagnetostriction. In particular, the [111] axis is preferable in theTb_(0.3)Dy_(0.7)Fe_(2.0) crystal, since it is an easy axis ofmagnetization. However, when x is small, the amount of Tb contained inthe raw material A is small, and this makes the orientation in the [111]axis direction difficult. And when x exceeds 0.5, the mixing ratio ofthe raw material C must be increased in order to obtain themagnetostrictive material represented by Expression 7, thus this reducesthe ratio of the raw material A, which decreases the degree oforientation in the [111] axis direction after sintering. For thesereasons, x should be in the range described above.

When y is less than 1.50, the mixing ratio of the raw material C must beincreased, which decreases the ratio of the raw material A, which inturn decreases the degree of orientation in the [111] axis directionafter sintering. When y is large, there are more phases rich in T suchas (Tb, Dy) T₃; this reduces the degree of orientation that results fromcompaction in magnetic field, and this in turn decreases the degree oforientation of the magnetostrictive material after sintering. For thesereasons, y should be in the range described above.

For the raw material B, a raw material represented by Expression 8:Dy_(t)T_(1-t) is used. Here, T is at least one type of metal selectedfrom among Fe, Co and Ni, and may be Fe alone. Part of Fe may bereplaced with Co or Ni, and when this is done the raw material B becomesmore easily ground and contributes to a higher sintering density aftersintering. Dy may include Tb or Ho, or may include both.

t in Expression 8 represents atomic ratio in the range of 0.37≦t≦1.00.Since Dy and T have an eutectic point, compositions in which t isoutside this range yield only a small amount of R₂T, which is theeutectic composition, in a mixture of the raw materials A and C, andthis makes it difficult to increase the sintering density.

In addition, it is preferable to perform hydrogen absorption treatmenton the raw material B, as indicated in FIG. 6. Rare earth metals such asDy are easily oxidized and therefore form oxide films in the presence ofeven a small amount of oxygen. An oxide film formed on the raw materialB impedes the progress of sintering in the subsequently sinteringprocess, and this causes the density of the sintered compact not toincrease. In view of this, the raw material B is made to absorb hydrogento make it less prone to becoming oxidized.

Further, by having the raw material B absorb hydrogen, strains arecaused in crystals as a result of hydrides forming or hydrogen atomspenetrating into the crystals. For this reason, when it is mixed withthe raw material A and the raw material C and pressure is applied toform a green compact, the pressure causes the destruction of the rawmaterial B particles to proceed; the raw material B particles in theirground and fine state can make their way in between the raw material Aparticles and contribute to forming a sintered compact with high densityin the subsequent sintering.

The amount of hydrogen to be absorbed by the raw material B should be inthe range of 7000 ppm to 22,000 ppm. When the amount of hydrogenabsorbed is under 7000 ppm, the internal strain of the raw material B issmall due to the small amount of hydrogen involved and this fails tolead to the destruction of the raw material B particles when compacting.And when the amount of hydrogen absorbed is over 22,000 ppm, the finepulverization of the raw material B particles reaches saturation andthere are no additional absorption effects.

For the raw material C, a raw material containing T is used. Asdiscussed earlier, T is a transition metal, preferably Fe, Co or Ni, andFe is the most preferred. Fe fuses during sintering to form anintermetallic compound with Tb or Dy and increases the magnetostrictionvalue.

The mixing proportions of the raw materials A, B and C may be determinedappropriately to obtain a magnetostrictive material as represented byExpression 6. The raw material A may be 50 wt. % or more but less than100 wt. %, preferably between 60 wt. % and 95 wt. %, against themagnetostrictive material. When there isn't enough raw material A, thereis not enough material to be oriented while compacting in a magneticfield, so that the degree of orientation of the magnetostrictivematerial obtained from sintering is low. And when there is too much rawmaterial A, the ratio of the raw material B that contains hydrogenbecomes small, which result in a failure to obtain a sintered compactwith high density.

The raw material B may be 40 wt. % or less, preferably between 5 wt. %and 30 wt. %, against the magnetostrictive material. When there isn'tenough raw material B, a sintered compact with high density cannot beobtained as described above. And when there is too much raw material B,the ratio of the raw material A becomes small and the degree oforientation fails to be enough.

The amount of the raw material C to be added is determined by takinginto account the proportions of the raw materials A and B and so that itwould correspond to the atomic ratio w of T in Expression 6.

The raw materials A, B and C are weighed out, mixed and ground, asindicated in FIG. 6. The average particle size of each of the rawmaterials should be 1-100 μm, more preferably 5-20 μm. When the averageparticle size is too small, the raw materials become oxidized duringmanufacture. Further, with small average particle size, themagnetization of the particles is also small, so that the magneticmoment of the magnetic field becomes small; this makes it difficult forthe particles to rotate, and this in turn makes it difficult to attain ahigh degree of orientation. When the average particle size is too large,sintering does not proceed smoothly and high sintering density cannot beachieved. On the other hand, when performing an HIP treatment, sinteringcan proceed even when the average particle size of powder is 10 μm ormore, as discussed earlier.

Grinding can be conducted by an appropriate grinding machine such as awet ball mill, attritor or atomizer. Of these, an atomizer isparticularly favorable. The reason for this is that since impact andshearing can be applied simultaneously with the atomizer, aggregation ofpowder can be prevented and there is little adhesion of powder to thedevice, which together lead to higher productivity.

After mixing, the mixture is compacted into a desired shape beforesintering By performing this compaction process in a magnetic field, theraw material A among other materials is aligned in a certain direction,so that the magnetostrictive material after sintering is oriented in the[111] direction. The magnetic field applied should be 24×10⁴ A/m ormore, preferably 48×10⁴ A/m or more. The direction of the magnetic fieldmay be either vertical or parallel to the direction of the pressureapplied. The compaction pressure should be 4.9×10⁴ Pa or more,preferably 2.9×10⁵ Pa or more.

Here, in the magnetostrictive material, the easy axis of magnetizationfor the raw material A is in the direction of the [111] axis, while itis in the direction of the [110] axis for the raw material B, and eachof the raw materials A and B has a large crystal magnetic anisotropy.Consequently, when compacting in a magnetic field, the direction of the[111] axis of the raw material A and the direction of the [110] axis ofthe raw material B become oriented in a direction parallel to themagnetic field. However, the raw material B acts as a fusing agentduring sintering: when it is made to contain hydrogen it becomes finelyground by the pressure applied during compaction in the magnetic field,which makes the raw material B more easily fused, so that it fuses withthe raw material C and forms an alloy with the raw material A, which hasbecome oriented in the direction of the [111] axis, and thereby forms amagnetostrictive material. Furthermore, by having the raw material Bbecome finely ground during compaction in the magnetic field, theparticles of the raw material A become more easily rotatable, whichmakes the latter more easily oriented towards the direction of themagnetic field.

The sintering conditions for the compact are not particularly limitedbut the temperature should be 1100° C. or higher, preferably 1150-1250°C., and should be done for one to ten hours. The atmosphere forsintering should be non-oxidized and should be an inert gas, such as Aror nitrogen, or vacuum atmosphere. Magnetostrictive materials withhigher density and with less internal strains can be obtained byrendering the sintering process and/or HIP treatment process discussedearlier in the first and second manufacturing methods for sinteredcompacts.

The present invention will be described in greater detail below withembodiment examples.

TEST EXAMPLE 1

For a raw material A, Tb, Dy and Fe were weighed out, fused in an inertgas atmosphere of Ar gas and an alloy was manufactured. Here, thecomposition was Tb_(0.4)Dy_(0.6)Fe_(1.94). A heat treatment to annealthe raw material A was rendered, the concentration distribution of eachof the metal elements during the manufacture of the alloy was madeuniform, the heterogeneous phases precipitated were eliminated, and theraw material A was ground with an atomizer.

For a raw material B, Dy and Fe were weighed out, fused in an inertatmosphere of Ar gas and an alloy was manufactured. Here, thecomposition was Dy_(2.0)Fe. The raw material B was ground with anatomizer in a manner similar to the raw material A.

For a raw material C, Fe that was reduction-treated to remove oxygen ina hydrogen gas atmosphere was used.

The raw materials A, B and C thus obtained were weighed out, ground andmixed, and an alloy powder with the compositionTb_(0.3)Dy_(0.7)Fe_(1.88) was compacted in a magnetic field of 80×10⁴A/m.

The temperature of the compacted alloy powder was raised in an oven andsintered in a mixed atmosphere of 35 vol. % hydrogen gas and 65 vol. %Ar gas in a stable temperature zone of 1150-1230° C., and the density ofthe resulting sintered compact is shown in FIG. 7. FIG. 7( a) indicatesthe temperature (° C.) of the heat treatment against the time passed (inminutes), and FIG. 7( b) indicates the density of the sintered compact(%) against the time passed (in minutes). Here, the density of thesintered compact represents the ratio of the specific gravity of thesintered compact to the absolute specific gravity of the alloy. Thezones indicated by black dots in FIGS. 7( a) and (b) are zones with amixed atmosphere of 35 vol. % hydrogen gas and 65 vol. % Ar gas, andother zones are an atmosphere of Ar gas alone. As shown in FIG. 7( b),the density of the sintered compact soars dramatically to above 90%immediately after the heat treatment in a mixed gas atmosphere. Further,by heat treating for approximately 180 minutes in a stable temperaturezone of 1225° C., the density of the sintered compact can reach 95% ormore.

Based on the above, we can see that a high density sintered compact canbe obtained by rendering a heat treatment in a mixed gas atmosphere ofhydrogen gas and Ar gas on the alloy powder molded.

TEST EXAMPLE 2

Next, the compacted alloy powder was sintered in a temperature zone of650-1238° C. as the volume ratio (concentration) of hydrogen gas in amixed gas of Ar gas and hydrogen gas was varied between 0 and 100 vol.%, and the density of the sintered compact and magnetostriction valuewere measured. The results are shown in Tables 1 and 2 and FIGS. 8 and9. Note, however, that the hydrogen storage material in Table 1 refersto a sintered compact that was made using an alloy powder in which oneof its raw materials, raw material B, was rendered a hydrogen absorptiontreatment, and the non-hydrogen storage material in Table 2 refers to asintered compact that was made using an alloy powder which had not beenrendered any hydrogen absorption treatments. The hydrogen absorptiontreatment was performed by having hydrogen absorbed in a hydrogen gasatmosphere while the temperature was maintained at a constanttemperature, and subsequently changing the atmosphere to an Ar gasatmosphere and maintaining this for a certain amount of time.

TABLE 1 Hydrogen Storage Material's Density of Sintered Compact andMagnetostriction value Hydrogen Storage Material Hydrogen ConcentrationDensity of Sintered Magnetostriction value (vol. %) Compact (%) (ppm) 088.1 1165 20 94.5 1164 30 96.3 1161 35 97.4 1164 40 97.8 1050 50 97.7880 100 98.1 660

From Table 1 and FIG. 8, we can see that the sintered compact accordingto the present invention shows gradual increases in its density as theconcentration of hydrogen gas increases, and in particular that thedensity of the sintered compact reaches 90% and higher when there iseven a small amount of hydrogen gas. However, the magnetostriction valueof the sintered compact falls when the concentration of hydrogen gasexceeds 35 vol. % and falls to 880 ppm, which is below 1000 ppm, whenthe concentration of hydrogen gas is 50 vol. % or more.

TABLE 2 Non-Hydrogen Storage Material's Density of Sintered Compact andMagnetostriction value Non-Hydrogen Storage Material HydrogenConcentration Density of Sintered Magnetostriction value (vol. %)Compact (%) (ppm) 0 84.5 1154 20 88.9 1161 30 90.5 1161 35 92.2 1165 4092.5 1090 50 93.1 890 100 94.4 680

From Table 2 and FIG. 9, we can see that the sintered compact made usinga non-hydrogen storage material also shows gradual increases in itsdensity as the concentration of hydrogen gas increases, and that thedensity of the sintered compact reaches 90% and higher when theconcentration of hydrogen gas is 30 vol. % or more. However, themagnetostriction value of the high density sintered compact falls whenthe concentration of hydrogen gas exceeds 35 vol. % and falls to 890ppm, which is below 1000 ppm, when the concentration of hydrogen gas is50 vol. % or more.

As a result, we can see that the concentration X of hydrogen gas maypreferably be less than 50 vol. % but greater than 0 vol. % when using araw material containing either the hydrogen storage material or thenon-hydrogen storage material.

TEST EXAMPLE 3

In a heat treatment of the sintered compact, the compacted alloy powderwas sintered in a temperature zone of 650-1225° C. as the atmosphere wasvaried from an Ar gas atmosphere to a mixed atmosphere of 35 vol. %hydrogen gas and 65 vol. % Ar gas, and the density of the sinteredcompact and magnetostriction value were measured. The raw material B wasrendered a hydrogen absorption treatment. The results are shown in Table3 and FIG. 10. In Table 3, the temperature at which the atmosphereshifted from a mixed atmosphere to an atmosphere of Ar gas alone is thehydrogenation ending temperature.

TABLE 3 Beginning Temperature of Mixed Gas Atmosphere, Density ofSintered Compact and Magnetostriction value 35 vol. %¹⁾ Hydrogenation 35vol. %¹⁾ Density of Magnetostriction Beginning Temp. HydrogenationSintered value (° C.) Ending Temp. (° C.) Compact (%) (ppm) 650 122597.5 860 950 1225 97.6 1030 1150 1225 97.4 1164 1200 1225 95.2 1158 12201225 93.1 1162 *¹⁾: Indicates a mixed gas atmosphere of hydrogen: Ar gas= 35 (vol. %): 65 (vol. %).

From Table 3 and FIG. 10, we can see that in a heat treatment of thesintered compact according to the present invention, the density of thesintered compact is extremely high with virtually all values at 97% ormore when the hydrogenation beginning temperature, at which a mixedatmosphere of hydrogen gas and Ar gas is created, is 650° C. or higher,but the density of the sintered compact gradually falls once thehydrogenation beginning temperature exceeds 1150° C. Themagnetostriction value grows larger as the hydrogenation beginningtemperature rises, but it reaches saturation when the hydrogenationbeginning temperature exceeds 1150° C.

As a result, we can see that hydrogen gas should be mixed at 650° C. atminimum to begin to create a mixed atmosphere of hydrogen gas and Argas.

TEST EXAMPLE 4

In a heat treatment of the sintered compact, the compacted alloy powderwas sintered as the atmosphere was varied from an Ar gas atmosphere to amixed atmosphere of 35 vol. % hydrogen gas and 65 vol. % Ar gas at thetemperature of 1150° C., and as the hydrogenation ending temperature, atwhich point the mixed gas atmosphere shifted to an atmosphere of Ar gasalone, was varied; and the density of the sintered compact andmagnetostriction value were measured. The raw material B was rendered ahydrogen absorption treatment. The results are shown in Table 4 and FIG.11.

TABLE 4 Ending Temperature of Mixed Gas Atmosphere, Density of SinteredCompact and Magnetostriction value 35 vol. %¹⁾ Hydrogenation 35 vol. %¹⁾Density of Magnetostriction Beginning Temp. Hydrogenation Sintered value(° C.) Ending Temp. (° C.) Compact (%) (ppm) 1150 1200 93.3 1163 11501220 95.5 1161 1150 1225 97.4 1164 1150 1230 98.2 1090 1150 1235 98.2920 1150 1235.5 98.3 880 *¹⁾: Indicates a mixed gas atmosphere ofhydrogen Ar gas = 35 (vol. %) 65 (vol. %).

From Table 4 and FIG. 11, we can see that the density of the sinteredcompact rises to 93.3% and higher when the hydrogenation endingtemperature is 1200° C. or higher, and that the density of the sinteredcompact increases as the ending temperature rises. However, themagnetostriction value of the sintered compact falls drastically whenthe ending temperature exceeds 1230° C. and falls to 920 ppm, which isbelow 1000 ppm, at 1235° C.

As a result, we can see that the hydrogenation ending temperaturepreferably should not exceed 1230° C., and more preferably 1220° C.

EMBODIMENT EXAMPLE 1 AND COMPARATIVE EXAMPLES 1-4

Compositions of sintered compacts in Embodiment Example 1 andComparative Examples 1-4 are shown in Table 5.

In Embodiment Example 1, a raw material A having the compositionTb_(0.4)Dy_(0.6)Fe_(1.94), a raw material B having the compositionDy_(2.0)Fe, and a raw material C in which Fe was reduction-treated toremove oxygen in a hydrogen gas atmosphere were weighed out, ground,mixed, and the alloy powder having the final compositionTb_(0.3)Dy_(0.7)Fe_(1.88) was formed by means of compaction in amagnetic field of 80×10⁴ A/m. Subsequently, it was sintered in an Ar gasatmosphere and in a mixed atmosphere of hydrogen gas and Ar gas, and asintered compact was manufactured. Sintering was conducted under theheat treatment conditions indicated in FIG. 7, so that a mixedatmosphere of 35 vol. % hydrogen gas and 65 vol. % Ar gas was created ina stable temperature zone and the atmosphere was subsequently changed toan Ar gas atmosphere. The raw material B was allowed to absorb hydrogenwhile the temperature was maintained at 150° C. in a hydrogen gasatmosphere, then the temperature was raised to 400° C. in a hydrogen gasatmosphere, and finally at 400° C. the atmosphere was changed to an Argas atmosphere, which was maintained for a certain amount of time toperform the hydrogen absorption treatment.

Comparative Example 1 is the same as the embodiment 1, includingcomposition, except that sintering was done in an atmosphere of Ar gasalone.

Comparative Example 2 is the same as the embodiment 1, includingcomposition, except that a raw material B that was not hydrogenabsorption-treated was used and that sintering was done in an atmosphereof Ar gas alone.

Comparative Example 3 is the same as the embodiment 1, except that a rawmaterial B that was not hydrogen absorption-treated was used and thatsintering was done in an atmosphere of Ar gas alone.

Comparative Example 4 is a sintered compact manufactured by the U.S.manufacturer ETREMA, using the single crystal growth method. Itscomposition is Tb_(0.3)Dy_(0.7)Fe_(1.93), which is generally the same asthe composition of Embodiment Example 1 and other examples.

The sintered compacts obtained from Embodiment Example 1 and ComparativeExamples 1-3 were each let stand for 1000 hours in atmospheric air at85° C., 100° C., 125° C., 155° C. and 200° C.; magnetostrictiveproperties were measured; and the deterioration rate of magnetostrictionvalue was measured for each. The relationship between the temperature(C) at which the sintered compacts were let stand and the deteriorationrate of magnetostriction value (%) is shown in FIG. 12. Thedeterioration rate of magnetostriction value for each sintered compactis expressed in terms of the ratio of the magnetostriction value at eachof the various temperatures to the initial magnetostriction value, whichis 100%.

TABLE 5 Composition and Other Information concerning Sintered Compactsin Embodiment 1 and Comparative Examples 1-4 Density of Sintered SampleNo. Conditions for Sintered Compact Symbol in FIG. 12 CompositionCompact (%) Embodiment Ex. Tb_(0.3)Dy_(0.7)Fe_(1.88) Used raw material Bthat 97 1 was hydrogen absorption- ● treated. Sintered in a mixedatmosphere of 35 vol. %¹⁾ hydrogen gas and Ar gas. Comparative Ex.Tb_(0.3)Dy_(0.7)Fe_(1.89) Used raw material B that 91 1 was hydrogenabsorption- Δ treated. Sintered in an atmosphere of Ar gas alone.Comparative Ex. Tb_(0.3)Dy_(0.7)Fe_(1.89) Did not use raw material 84 2B that was hydrogen □ absorption-treated. Sintered in an atmosphere ofAr gas alone. Comparative Ex. Tb_(0.3)Dy_(0.7)Fe_(1.89) Did not use rawmaterial 84 3 B that was hydrogen ∘ absorption-treated. Sintered in anatmosphere of Ar gas alone. Comparison Ex. 4 Tb_(0.3)Dy_(0.7)Fe_(1.93)Single crystal 99.5 ▪ (manufactured by ETREMA). *¹⁾: Indicates a mixedgas atmosphere of hydrogen: Ar gas = 35 (vol. %): 65 (vol. %).

As we can see from Table 5 and FIG. 12 and from comparing EmbodimentExample 1 to Comparative Example 1, the density of sintered compact canbe increased from 91% to 97% by using a mixed atmosphere of hydrogen gasand Ar gas. As a result of this, when sintered compacts are let standfor 1000 hours at a high temperature of 200° C., the deterioration ratesof magnetostriction value for Embodiment Example 1 are 90% or more,while the deterioration rate for Comparative Example 1 falls to lessthan 90%, as shown in FIG. 12. This shows that by increasing the densityof sintered compact, the deterioration rate of magnetostriction valuecan be maintained at a high rate and the deterioration suppressed.

When part of raw materials is not hydrogen absorption-treated and theheat treatment during sintering is conducted in an atmosphere of Ar gasalone instead of a mixed atmosphere of hydrogen gas and Ar gas, as inComparative Examples 2 and 3, we can see that the density of thesintered compact in each case is low at 84%. Further, while themagnetostriction value of Embodiment Example 1 hardly shows any dropwhen let stand at high temperatures of 125° C. and 155° C., themagnetostriction value of Comparative Example 2 drops to 95% when letstand at a high temperature of 125° C., and that of Comparative Example3 drops to 90%; when let stand at a high temperature of 155° C., themagnetostriction value of Comparative Example 2 drops dramatically to88% and that of Comparative Example 3 to 81%. We can see from these thatbecause the density of sintered compact can be increased by in a mixedatmosphere of hydrogen gas and Ar gas during sintering than in anatmosphere of Ar gas alone, the deterioration of the magnetostrictionvalue of magnetostrictive material can be restricted.

Furthermore, when Embodiment Example 1 and Comparative Example 4 arecompared, we can see that Comparative Example 4 is a single crystal witha density of 99.5%, which is very close to 100%, and that EmbodimentExample 1's deterioration rate of magnetostriction value is equivalentto that of the single crystal Comparative Example 4. Based on the above,we can see that by using the method for manufacturing sintered compactaccording to the present invention, a sintered compact ormagnetostrictor with properties virtually equivalent to those of asingle crystal can be manufactured at low cost and in shapes desiredthrough the use of metal molds that allows freedom in the selection ofshapes.

EMBODIMENT EXAMPLE 2

For a raw material A, Tb, Dy and Fe were weighed out and fused in an Argas atmosphere to make an alloy having the compositionTb_(0.4)Dy_(0.6)Fe_(1.93). For a raw material B, Dy was made to absorbhydrogen to make DyH₂. For a raw material C, Fe was used. The rawmaterials A, B and C were mixed and ground with an atomizer in an Ar gasatmosphere. The average particle size was measured using a subsievesizer (by Fisher).

Next, the resulting powder was compacted into a cylindrical shape withdimensions of 3.5×30 mm in a magnetic field of 80×10⁴ A/m. Thetemperature of the green compact was raised in an Ar gas atmosphere andwhen the temperature reached 600° C., the green compact was sintered invacuum. The temperature profile for this process is shown in FIG. 13.The sintered compact was then HIP-treated. The temperature and pressureprofiles for this process are shown in FIG. 14. The HIP-treated productwas then annealed. The temperature profile for this process is shown inFIG. 15.

The relative density and magnetostriction value of the sintered compactthus manufactured through these processes are shown in Table 6.

The relative density of the sintered compact is expressed as the ratio(%) of each sample's dimensions measured to true density. A strain gaugewas affixed to a sample in the shape of a cylinder with dimensions of3.5×30 mm, the sample is placed in a magnetic field and themagnetostriction value was measured.

COMPARATIVE EXAMPLE 5

Comparative Example 5 was manufactured in the same way as EmbodimentExample 2 except that the green compact was sintered in an Ar gasatmosphere.

TABLE 6 Relative Densities and Magnetostriction values of SinteredCompacts Evaluation Items Rel. Density of Sample No. Sintered CompactMagnetostriction Atmosphere Process Step (%) value (Δl/l ppm) EmbodimentAfter Sintering 88.7 1001 Ex. 2 After HIP 99.1 890 Vacuum Treatment 99.11012 Sintering After Annealing Embodiment After Sintering 89.1 1130 Ex.5 After HIP 98.3 871 Sintering in Treatment 93.5 1073 Ar After AnnealingAtmosphere

FIGS. 16 and 17 are photographs of interiors of sintered compacts inEmbodiment Example 2 and Comparative Example 5, respectively, seenthrough a microscope and photographed. FIGS. 16( a) and 17(a) arephotographs of the interiors of sintered compacts after sintering, FIGS.16( b) and 17(b) are photographs of the interiors of sintered compactsafter HIP treatment, and FIG. 16( c) and 17(c) are photographs of theinteriors of sintered compacts after annealing.

Table 6 shows that densities of the sintered compacts in EmbodimentExample 2 and Comparative Example 5 are virtually the same. We can seethis also by comparing FIG. 16( a) and FIG. 17( a), which show thatblack pore parts are equivalent and numerous.

However, in Embodiment Example 2, the density of the sintered compactincreased after the HIP treatment and also after annealing, both ofwhich followed sintering, and the value is 99.1% for both, which ishigher than 98%. This can be observed on the FIG. 16( b) and FIG. 16(c), which show that there are hardly any black pore parts. Further,since the magnetostriction value after annealing is higher than themagnetostriction value after the HIP treatment, we can see that there islittle internal strain.

In Embodiment Example 2, the sintered compact after sintering in somecases showed a significant drop in the magnetostriction value after along-term use due to the high number of pores, and cracks sometimesappeared as a result of repeated occurrences of internal stress due torepeated use.

Comparative Example 5's relative density is over 98% at 98.3% after theHIP treatment following sintering, but its magnetostriction value isnearly 20 ppm lower than that of Embodiment Example 2. Further,Comparative Example 5's magnetostriction value is high but its densityis a low 93.5% after annealing, and numerous cracks appeared afterannealing. This can be observed on FIG. 17( b) and FIG. 17( c): althoughthere are hardly any pore parts after the HIP treatment, residual Ar gasinside closed pores expanded during annealing and the closed pores havegrown larger. This is thought to have led to larger internal strain,which then led to cracks. In Comparative Example 5 the sintered compactafter sintering showed a drastic fall in the magnetostriction valueafter a long-term use, due to the high number of pores.

From the above, we can see that high density and magnetostriction valuecan be obtained for the sintered compact by sintering in vacuum andthrough subsequent HIP treatment and/or annealing.

EMBODIMENTS EXAMPLES 3 AND 4

Sintered compacts in Embodiment Examples 3 and 4 were manufactured inthe same way as Embodiment Example 2, except that sintering wasconducted in a mixed gas atmosphere of hydrogen:Ar=100:0 for EmbodimentExample 3 and in a mixed gas atmosphere of hydrogen:Ar=65:35 forEmbodiment Example 4.

COMPARATIVE EXAMPLES 6 AND 7

Sintered compacts in Comparative Examples 6 and 7 were manufactured inthe same way as the embodiment 2, except that sintering was conducted ina mixed gas atmosphere of hydrogen:Ar=50:50 for Comparative Example 6and in a mixed gas atmosphere of hydrogen:Ar=35:65 for ComparativeExample 7.

Relative densities and magnetostriction values of sintered compacts ofmagnetostrictive materials manufactured thus are shown in Table 7.

TABLE 7 Relative Densities and Magnetostriction values of SinteredCompacts Evaluation Items Rel. Density Sample No. of SinteredMagnetostriction Atmosphere Process Step Compact (%) value (Δl/l ppm)Embodiment Ex. 3 After Sintering 96.8 968 Hydrogen:Ar = 100:0 After HIP99.1 830 Treatment 99.2 963 After Annealing Embodiment Ex. 4 AfterSintering 97.1 1020 Hydrogen:Ar = 65:35 After HIP 99.4 954 Treatment98.7 1001 After Annealing Comparative Ex. 6 After Sintering 96.5 1008Hydrogen:Ar = 50:50 After HIP 98.7 813 Treatment 96.8 967 AfterAnnealing Comparative Ex. 7 After Sintering 95.6 1050 Hydrogen:Ar =35:65 After HIP 98.4 897 Treatment 95.7 1033 After Annealing

FIGS. 18, 19, 20 and 21 are interiors of sintered compacts of EmbodimentExample 3, Embodiment Example 4, Comparative Example 6 and ComparativeExample 7, respectively, seen through a microscope and photographed.FIGS. 18( a), 19(a), 20(a) and 21(a) are photographs of interiors of thesintered compacts after sintering, FIGS. 18( b), 19(b), 20(b) and 21(b)are photographs of interiors of the sintered compacts after HIPtreatment, and FIGS. 18( c), 19(c), 20(c) and 21(c) are interiors ofsintered compacts after annealing.

We can see from Table 7 that in Embodiment Example 3, the densities ofthe sintered compact after HIP treatment and after annealing arevirtually the same and that most of the residual hydrogen in pores haddissipated. This is obvious also by comparing photographs in FIG. 18( b)and FIG. 18( c), which show that the proportions of pores are nearly thesame in the two photographs. In Embodiment Example 4, Ar gas constitutes35% of the atmosphere, and this causes the density of the sinteredcompact after annealing to be 98.7%, which is slightly lower than the99.4% after the HIP treatment. This is a result of residual Ar gas thatcould not dissipate and that expanded. However, the magnetostrictionvalue is larger after annealing since annealing removed residualstrains.

In Comparative Examples 6 and 7, we can see that although theirmagnetostriction values after annealing are larger than after the HIPtreatment, their densities of the sintered compacts are clearly lowerafter annealing than after the HIP treatment due to the impact of theresidual Ar gas. In Comparative Example 6, the sintered compact aftersintering showed a significant drop in the magnetostriction value aftera long-term use and cracks appeared while it was being used as amagnetostrictor. Comparative Example 7 showed numerous cracks afterannealing.

EMBODIMENT EXAMPLE 5

For a raw material A, Tb, Dy and Fe were weighed out, fused in an inertatmosphere of Ar gas, and an alloy having the compositionTb_(0.4)Dy_(0.6)Fe_(1.93) was made. The alloy was heat treated forannealing, the concentration distribution of each of the metal elementswhen making the alloy was made uniform, and the heterogeneous phasesthat precipitated were eliminated. Next, the raw material A was ground.

For a raw material B, Dy and Fe were weighed out and fused in an inertatmosphere of Ar gas, and an alloy having the compositionDy_(2.0)Fe_(1.0) was made. Next, the alloy was ground. After that, theraw material B was held in a mixed atmosphere of hydrogen and Ar gasesand hydrogen absorption-treated.

For a raw material C, Fe that was reduction-treated to remove oxygen ina hydrogen gas atmosphere was used.

Next, the raw materials A, B and C were weighed out separately and mixedand ground. An atomizer (by Tokyo Atomizer Manufacturing Co., Ltd.) wasused for grinding.

Following this, the mixture was compacted in a magnetic field. Thecompaction was performed under the following conditions against a12×12×16 mm³ sample in the shape of a square pillar: the direction ofthe magnetic field was parallel to the axial direction, the magneticfield to be applied was 72×10⁴ A/m, and the compaction pressure was8640×10⁴ Pa to form a green compact. The green compact was sintered inan Ar atmosphere and a magnetostrictive material was obtained.

The magnetostriction value was measured by applying a magnetic field of8.0×10⁴ A/m to the magnetostrictive material and measuring the resultingstrain with a strain gauge.

COMPARATIVE EXAMPLE 8

In Comparative Example 8, the magnetostrictive material was manufacturedin the same way as Embodiment Example 5, except that after the rawmaterials A, B and C were mixed and ground, they were compacted withouthaving any magnetic fields applied to them.

Table 8 shows the strengths of magnetic fields applied, the degrees oforientation after sintering, and the magnetostriction values.

TABLE 8 Strengths of Magnetic Fields Applied, Degrees of Orientationafter Sintering, and Magnetostriction values Embodiment No. ComparativeEx. Magnetic Field Degree of Magnetostriction No. Applied (10⁴ A/m)Orientation value λ_(1.0) (ppm) Embodiment 5 24 4.0 800.0 Comparative 01.0 600.0 Ex. 8

As Table 8 makes clear, Comparative Example 8 that did not have anymagnetic fields applied to it has a low degree of orientation and lowmagnetostriction value λ_(1.0). From this, we can see that mixing andgrinding the raw materials A, B and C and applying a magnetic field leadto a higher degree of orientation and a higher magnetostriction valueλ_(1.0) for magnetostrictive material. In addition, we can see that whenthe degree of orientation is 4.0, which is higher than 2.0, amagnetostriction value λ_(1.0) of 800 ppm, which is higher than 700 ppm,can be obtained.

EMBODIMENT EXAMPLES 6-8 AND COMPARATIVE EXAMPLE 9

Embodiment Examples 6-8 and Comparative Example 9 were manufactured inthe same way as Embodiment Example 5, except that the strength of themagnetic field applied when compacting in the magnetic field was varied.

Table 9 shows the strength of magnetic field applied, the degree oforientation after sintering, and the magnetostriction value for each.

TABLE 9 Strengths of Magnetic Fields Applied, Degrees of Orientationafter Sintering, and Magnetostriction values Embodiment Ex. No.Comparative Magnetic Field Degree of Magnetostriction Ex. No. Applied(10⁴ A/m) Orientation value λ_(1.0) (ppm) Embodiment 24 4.0 800.0 Ex. 5Embodiment 48 10.0 950.0 Ex. 6 Embodiment 64 13.0 1050.0 Ex. 7Embodiment 80 20.0 1220.0 Ex. 8 Comparison 16 1.9 650.0 Ex. 9

As Table 9 clearly shows, the magnetic field applied causes the degreeof orientation to change; in Comparative Example 9 the magnetostrictionvalue λ_(1.0) is 650 ppm when the degree of orientation is 1.9, and inorder to obtain a magnetostriction value λ_(1.0) of 700 ppm or more, thedegree of orientation must be higher than that. Further, as we can seefrom Embodiment Examples 6-8, when the degree of orientation is 7.0 ormore the magnetostriction value λ_(1.0) of 900 ppm can be obtained.

EMBODIMENT EXAMPLE 9

For a raw material A, Tb, Dy and Fe were weighed out and fused in aninert atmosphere of Ar gas to make an alloy Tb_(0.4)Dy_(0.6)Fe_(1.93).Next, the alloy was annealed and ground. First, it was coarsely groundwith a jaw crusher, then finely ground with a Brown mill until theaverage particle size was 100-1500 μm.

For a raw material B, Dy and Fe were weighed out and fused in an inertatmosphere of Ar gas to make an alloy Dy_(2.0)Fe_(1.0). The alloy wasground with a jaw crusher until the average particle size was 2-10 mm.Next, the ground particles were held in a mixed atmosphere of hydrogenand Ar gases and hydrogen absorption-treated.

For a raw material C, reduced iron with average particle size ofapproximately 5 μm was used. The reduced iron was held for approximately30 minutes at approximately 200° C. in a hydrogen gas atmosphere for areduction treatment to remove oxygen.

Next, the raw materials A, B and C were weighed out to result with thecomposition Tb_(0.3)Dy_(0.7)Fe_(1.88). These were mixed, then a grindingmachine was used to further grind and mix them. Here, an atomizer (byTokyo Atomizer Manufacturing Co., Ltd.) was used as the grinding machineto grind until the average particle size was approximately 15 μm.

Thereafter, the mixture was compacted in a magnetic field of 80×10⁴(A/m) in parallel direction and under a pressure of 59×10⁷ Pa to form agreen compact.

Next, the green compact was sintered in an Ar gas atmosphere and amagnetostrictive material was manufactured. The heat treatmentconditions for sintering were the following: raise the temperature to940° C. at the rate of 5° C./min. and hold for one hour; and once thegreen compact's temperature became uniform, the temperature was raisedto 1235° C. and held there for three hours; the sintering was completedand a magnetostrictive material was obtained.

An SEM photograph of the magnetostrictive material obtained inEmbodiment Example 9 is shown on FIG. 22. When a chemical compositionanalysis was performed using an energy dispersion x-ray spectrometer(EDS) on each of the phases found in the cross-section, phases indicatedby 1 and 2 were found to be the main phase having the composition (Tb,Dy) Fe₂. The phase indicated by 3 was found to be a phase whose primarycomponents are Tb and Dy.

Next, we looked for the ratio ([R]/[RT₂]) of the phase whose primarycomponents are Tb and Dy to the main phase based on the area of each ofthe phases, and found the ratio to be 0.0015. When the magnetostrictionvalue of this magnetostrictive material in a magnetic field of 8×10⁴ A/mwas measured, the value was a high 1200 ppm.

EMBODIMENT EXAMPLE 10

Except that raw materials A, B and C were weighed out to result with thecomposition Tb_(0.3)Dy_(0.7)Fe_(1.92), the mixture was compacted in amagnetic field and sintered in the same way as Embodiment Example 9 toobtain a magnetostrictive material.

An SEM photograph of the cross-section of the magnetostrictive materialobtained is shown on FIG. 23. When a chemical composition analysis wasperformed using an EDS on each of the phases found in the cross-section,the phases indicated by 1 and 2 were found to be the main phase havingthe composition (Tb, Dy) Fe₂. The phase indicated by 3 was found to be aphase whose primary components are Tb and Dy.

Next, we looked for the ratio ([R]/[RT₂]) of the phase whose primarycomponents are Tb and Dy to the main phase based on the area of each ofthe phases, and found the ratio to be 0.0072. When the magnetostrictionvalue of this magnetostrictive material in a magnetic field of 8×10⁴ A/mwas measured, the value was a high 1030 ppm.

INDUSTRIAL APPLICABILITY

By using the methods for manufacturing sintered compacts according tothe present invention, fine sintered compacts with high density can beobtained. Further, by reducing the oxide content during the manufactureof sintered compact and thereby having closed pores with small internalpressure, a sintered compact with little internal strain can bemanufactured. Moreover, by applying the methods for manufacturingsintered compacts according to the present invention to the manufactureof magnetostrictive material, a magnetostrictive material with a largemagnetostriction value, small reduction in magnetostriction value overtime, and without cracks can be obtained.

In addition, by increasing the degree of orientation in the [111] axisdirection, which is an easy axis of magnetization and has largemagnetostrictive constant, a magnetostrictive material with highmagnetostriction value can result. And by stipulating the ratio ofphases whose primary components are R to the main phase RT2, a superiormagnetostrictive material with no fluctuations in properties amongproducts can be provided.

1. A magnetostrictive material: formed by compaction in a magnetic fieldand then sintering a mixture containing a material A that is representedby Expression 7: (Tb_(x)Dy_(1-x))T_(y)Ty (where, x and y atom ratioswherein x and y are in ranges of 0.35 <x <0.50 and 1.50 <y <2.30,respectively), a material B that is represented by Expression 8:Dy_(t)T_(1-t) (where, Dy include at least one of Tb and Ho, and t is anatom ratio in range of 0.37 <t <1.00) and that contains hydrogen in theamount from 7000 ppm to 22000 ppm, and a material C containing T, andwherein the magnetostrictive material comprises: RT_(w)(where, R is atleast one kind of rare earth metal, T is at least one kind of transitionmetal, and w is 1.50 ≦w ≦2.30); and a degree of orientation greater orequal to 2; wherein the degree of orientation is$\frac{{I(222)}( \text{//} )\text{/}{I(311)}( \text{//} )}{{I(222)}(\bot)\text{/}{I(311)}(\bot)}$(where, each of the I (222) and I (311) represents an x-ray diffractionintensity on a (222) plane and a (311) plane, respectively, and (//) and(⊥) represent measurements taken on a plane parallel and on a planevertical, respectively, to a magnetic field orientation in thecompaction in the magnetic field).
 2. A magnetostrictive materialaccording to claim 1, wherein the degree of orientation of themagnetostrictive material in which the [111] axis orients in a directionin parallel with the magnetic field has a value of 7.0 or greater giventhe degree of orientation.
 3. A magnetostrictive material: formed bycompaction in a magnetic field and then sintering a mixture containing amaterial A that is represented by Expression 7: (Tb_(x)Dy_(1-x))T_(y)Ty(where, x and y are atom ratios wherein x and y are in ranges of 0.35 <x<0.50 and 1.50 <y <2.30, respectively), a material B that is representedby Expression 8: Dy_(t)T_(1-t) (where, Dy include at least one of Tb andHo, and t is an atom ratio in range of 0.37 <t <1.00) and that containshydrogen in the amount from 7000 ppm to 22000 ppm, and a material Ccontaining T, and wherein the magnetostrictive material comprises:RT_(w)(where, R is at least one kind of rare earth metal, T is at leastone kind of transition metal, and w is 1.50 ≦w ≦2.30); a structurecomposed of a RT₂main phase and at least one kind of heterogeneous phaseincluding a phase having R as a main composition; wherein a ratio of thephase having R as a main composition among the heterogeneous phase tothe RT₂main phase [R]/[RT₂]is in a range of 0 <[R]/[RT2]≦0.45.
 4. Amagnetostrictive material according to any one of claims 1 through 3,wherein the magnetostrictive material is represented by Expression 6:(Tb_(x)Dy_(1-x))T_(w)T_(w)(where, v and w are atom ratios wherein v andw are in ranges of 0.27 ≦v ≦0.50 and 1.50 ≦w ≦2.30, respectively).
 5. Amagnetostrictive material according to any one of claims 2 or 3, whereinT in the magnetostrictive material is at least one kind of metalselected from Fe, Ni and Co.
 6. A magnetostrictive material according toclaim 4 wherein T in the magnetostrictive material is at least one kindof metal selected from Fe, Ni and Co.
 7. A magnetostrictive material,according to claim 4 wherein the magnetostrictive material is formed bycompacting in a magnetic field and then sintering a mixture containing amaterial A that is represented by Expression 7: (Tb_(x)Dy_(1-x))T_(y)Ty(where, x and y are atom ratios wherein x and y are in ranges of 0.35 <x≦0.50 and 1.50 ≦y ≦2.30, respectively), a material B that is representedby Expression 8: Dy_(t)T_(1-t)(where, Dy includes at least one of Tb andHo, and t is an atom ratio in a rage of 0.37 ≦t ≦1.00), and a material Ccontaining T.
 8. A magnetostrictive material according to claim 5,wherein the magnetostrictive material is formed by compacting in amagnetic field and then sintering a mixture containing a material A thatis represented by Expression 7: (Tb_(x)Dy_(1-x)) T_(y)(where, x and 7are atom ratios wherein x and y are in ranges of 0.35 <x ≦0.50 and 1.50≦y ≦2.30, respectively), a material B that is represented by Expression8: Dy_(t)T_(1-t)(where, Dy includes at least one of Tb and Ho, and t isan atom ratio in a rage of 0.37 ≦t ≦1.00), and a material C containingT.
 9. A magnetostrictive material according to claim 6, wherein themagnetostrictive material is formed by compacting in a magnetic fieldand then sintering a mixture containing a material A that is representedby Expression 7: (Tb_(x)Dy_(1-x))T_(y)Ty (where, x and y are atom ratioswherein x and y are in ranges of 0.35 <x ≦0.50 and 1.50 ≦y ≦2.30,respectively), a material B that is represented by Expression 8:Dy_(t)T_(1-t)(where, Dy includes at least one of Tb and Ho, and t is anatom ratio in a range of 0.37 ≦t ≦1.00), and a material C containing T.10. A magnetostrictive material according to claim 7, wherein thematerial B represented by Expression 8 in the magnetostrictive materialcontains hydrogen in the amount from 7000 ppm to 22000 ppm.
 11. Amagnetostrictive material according to claim 8, wherein the material Brepresented by Expression 8 in the magnetostrictive material containshydrogen in the amount from 7000 ppm to 22000 ppm.
 12. Amagnetostrictive material according to claim 9, wherein the material Brepresented by Expression 8 in the magnetostrictive material containshydrogen in the amount of 7000 ppm or greater but 22000 ppm or lower.13. The method of manufacturing a magnetostrictive material comprises,compacting in a magnetic field and then sintering a mixture containing amaterial A that is represented by Expression 7: (Tb_(x)Dy_(1-x))T_(y)Ty(where, x and y are atom ratios wherein x and y are in ranges of 0.35 <x<0.50 and 1.50 <y <2.30, respectively), a material B that is representedby Expression 8: Dy_(t)T_(1-t)t (where, Dy include at least one of Tband Ho, and t is an atom ratio in range of 0.37 <t <1.00) and thatcontains hydrogen in the amount from 7000 ppm to 22000 ppm, and amaterial C containing T.